Zstd: Browser Support, Algorithm, vs Brotli

What is Zstd?

Zstd, short for Zstandard, is a fast lossless compression algorithm that Yann Collet wrote at Meta and the IETF standardized as RFC 8878. It pairs LZ77 dictionary matching with two entropy coders, ships as the application/zstd MIME type and the .zst file extension, and powers HTTP content negotiation through the Accept-Encoding: zstd header.

Which browsers does Zstd support?

Zstd is supported in modern Chromium-based browsers and recent Firefox builds, with Safari joining on Safari 26 and Internet Explorer, Opera Mobile, and Samsung Internet still without support.

How does Zstd compression work?

Zstd combines LZ77 dictionary matching with two entropy coders to deliver decompression speeds above 500 MB/s per core while still beating Gzip on size. The encoder runs in two stages, then exposes a wide compression-level dial that servers can tune for the workload. Three building blocks drive the savings:

• LZ77 backreferences: Zstd scans the input for repeated byte sequences and replaces later copies with short pointers back to the first match in a sliding window of up to 128 MB at the highest levels.
• Huff0 entropy coder: Zstd rewrites literal bytes with a bit-packed Huffman coder named Huff0 that swaps tables on the fly, so frequent symbols use fewer bits than rare ones.
• Finite State Entropy (FSE): Zstd encodes match offsets and lengths with a fast asymmetric numeral systems (ANS) coder called FSE, which gets close to Huffman compression at twice the speed.
• Trained dictionaries: For short, similar payloads such as JSON API responses, Zstd accepts a pre-trained dictionary that primes the LZ77 window and shrinks small messages by up to 5 to 10 times.

Zstd exposes 22 standard compression levels from 1 (fastest) to 22 (smallest output), plus a --fast=N range below level 1 for negative levels. Most servers run levels 3 to 6 on dynamic responses, level 19 on static assets compressed at build time, and the dictionary mode for batches of similarly shaped JSON or protobuf payloads.

How do you enable Zstd on your server?

Zstd is on by default in Cloudflare, Fastly, and most major CDNs, but you may need to compile or wire it up yourself for Nginx, Apache, or a custom origin. Follow these steps to add Zstd to Nginx through the ngx_zstd module:

• Install the Nginx Zstd module: Clone tokers/zstd-nginx-module from GitHub and rebuild Nginx with --add-dynamic-module=/path/to/zstd-nginx-module, or install the prebuilt nginx-module-zstd package on your distro.
• Load the modules in nginx.conf: Add load_module modules/ngx_http_zstd_filter_module.so; and load_module modules/ngx_http_zstd_static_module.so; at the top of nginx.conf, above the events block.
• Configure compression in a server block: Inside the server block, add zstd on;, zstd_comp_level 6;, and a zstd_types line listing MIME types such as text/css, application/javascript, application/json, text/html, and image/svg+xml.
• Reload Nginx: Run sudo nginx -t to confirm the config parses, then sudo nginx -s reload so the new directives take effect without dropping live connections.
• Confirm Zstd is active: From your terminal, run curl -I -H "Accept-Encoding: zstd" https://your-site.com/ and look for content-encoding: zstd in the response headers.

If the response still comes back as content-encoding: gzip, the Zstd module did not load or the request URL is on plain HTTP. Re-check the load_module lines and confirm the request used HTTPS.

What are the known issues with Zstd?

Zstd is the fastest mainstream HTTP compression format, but a few real edge cases still bite in production. The biggest hits are HTTPS-only behavior, narrow browser reach below the modern stable lines, and uneven origin-server defaults.

• HTTPS-only in browsers: Chrome, Edge, Firefox, and Safari only advertise Accept-Encoding: zstd over HTTPS. Plain HTTP responses fall back to Gzip even when both client and server support Zstd.
• Narrow Safari and iOS coverage: Safari 25 and earlier on macOS and iOS 18.7 and earlier do not support Zstd. iPhones and iPads on iOS 18 still fall back to Brotli, so Brotli has to stay in the negotiation chain for at least one more major iOS cycle.
• Samsung Internet has not flipped the flag: Samsung Internet 29 still does not advertise zstd in Accept-Encoding, even though the underlying Chromium ships it. Galaxy traffic falls back to Brotli on every request.
• Origin servers default to Gzip or Brotli: Nginx, Apache, IIS, and Tomcat ship Gzip on by default and require manual setup for Zstd, so dev and staging environments often serve Gzip and mask the real-world payload sizes a CDN will produce.
• CDN re-compression varies by tier: Some CDNs strip Accept-Encoding: zstd on the way to the origin and re-compress at the edge at a lower level. Static assets miss out on the level-19 savings unless you precompress and serve .zst files directly.
• No DecompressionStream support: The DecompressionStream API supports gzip, deflate, and deflate-raw widely but does not include zstd, so client-side Zstd decoding still needs a WebAssembly polyfill such as zstd-wasm in every browser.

In my experience, the most surprising failure is a CDN silently downgrading Zstd to Brotli for Safari clients while a single Accept-Encoding header parser at the edge gets confused by zstd, br appearing together. The fix is to log the Vary: Accept-Encoding header on a cache-miss and confirm the edge actually served zstd to Chrome and Brotli to Safari, rather than serving Brotli to both.

Citations

All Zstd version numbers and platform notes in this guide come from these primary sources:

• MDN Web Docs - Zstandard compression: developer.mozilla.org/en-US/docs/Glossary/Zstandard_compression
• MDN Web Docs - Content-Encoding header: developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Content-Encoding
• IETF RFC 8878 - Zstandard Compression: datatracker.ietf.org/doc/html/rfc8878
• Wikipedia - Zstandard: en.wikipedia.org/wiki/Zstd
• Zstandard reference implementation - GitHub: github.com/facebook/zstd
• Zstandard project site: facebook.github.io/zstd
• Chrome Platform Status - Zstd Content-Encoding: chromestatus.com/feature/6186023867908096
• Chrome blog - Intent to Experiment, Zstd Content-Encoding: groups.google.com/a/chromium.org/g/blink-dev/c/I6IWfl95gRU
• Microsoft Learn - ZstdContentEncodingEnabled policy: learn.microsoft.com/en-us/deployedge/microsoft-edge-browser-policies/zstdcontentencodingenabled
• Bugzilla - Mozilla Implement support for Zstandard: bugzilla.mozilla.org/show_bug.cgi?id=1301878
• Mozilla - Firefox 126 release notes: mozilla.org/en-US/firefox/126.0/releasenotes
• Nginx Zstd module - GitHub: github.com/tokers/zstd-nginx-module
• zstd-wasm - GitHub: github.com/bokuweb/zstd-wasm